Yes. Many times. It happens often enough that we know almost all the reasons, too. Enlarged Heart Syndrome is often the cause of these world class athletes collapsing and dying right on the field/court. Other reasons are steroid use (elevates your hematocrit count which means it thickens your blood), erythropoietin doping, illegal stimulants (meth, cocaine), dehydration (no excuse with all the modern science we have available, for this happening), and heat exhaustion (can happen in ice cold weather, in sports).